ath11k: fix rcu lock protect in peer assoc confirmation

ath11k_mac_get_ar_by_vdev_id() get protected under rcu lock
and unlock. peer association confirmation event get used this API
without rcu protection, so corrected it.

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>